The present invention relates to a core for an induction coil, an illumination unit using the same, and a polycrystalline ferrite. More particularly, the present invention relates to a core for an induction coil used for a discharge lamp that generates an electromagnetic field inside a bulb, which contains discharge gas therein, by means of the induction coil.
FIG. 7 schematically illustrates a cross-sectional construction of an illumination unit that generates plasma discharge by means of an induction coil. The illustrated illumination unit includes: a translucent bulb 101 containing discharge gas such as inert gas and metal vapor inside; and an induction coil 102 placed in a cavity 108 formed in the center of the bulb 101. The induction coil 102 is essentially composed of a core (magnetic material) 102a and a winding 102b wound around the core 102a. The induction coil 102 is electrically connected to a high-frequency power supply circuit 103 for supplying high-frequency power via a matching circuit 104. The matching circuit 104 is provided to match the impedance between the induction coil 102 and the high-frequency power supply circuit 103 for efficient transfer of high-frequency power to the bulb 101. The high-frequency power supply circuit 103 and the matching circuit 104 are housed in a circuit case 105.
When a high-frequency current in the range of several megahertz to several hundred megahertz is applied to the induction coil 102 from the high-frequency power supply circuit 103, ring-shaped plasma discharge 106 is generated inside the bulb 101. Generation of the plasma discharge 106 causes emission of ultraviolet light or visible light, whereby light output is obtained. In practice, the oscillating frequency of the high-frequency power supply circuit 103 is 13.56 MHz or several megahertz in the ISM band.
An example of the core 102a suitably usable for the illumination unit described above is disclosed in Japanese Laid-Open Patent Publication No. 7-99042. The publication describes a core having a loss of 150 mW/cm3 or less when measured at room temperature with a frequency of 3 MHz and a magnetic flux density of 10 mT, and discloses that Nixe2x80x94Zn ferrite material is suitable as the material satisfying this condition.
When Nixe2x80x94Zn ferrite material is used for the core 102a, it is preferable to set the operating frequency of the high-frequency power supply circuit 103 at a high frequency of several megahertz (or in the range of several megahertz to several tens of megahertz) from the standpoint of the physical properties of the Nixe2x80x94Zn ferrite material. In order to realize such high-frequency operation, however, a large-size noise filter is required to suppress line noise generated from the high-frequency power supply circuit 103. This disadvantageously increases the volume of the high-frequency power supply circuit 103. In other words, the illumination unit using the core 102a made of a Nixe2x80x94Zn ferrite material has a problem that a large-size noise filter is additionally required and thus downsizing of the illumination unit is difficult.
Moreover, in the case where high-frequency noise is radiated or propagated from the unit, extremely strict regulations are imposed for prevention of the noise. In order to satisfy these regulations, therefore, an expensive noise filter must be used for the illumination unit. This poses a big barrier against attempt of cost reduction of the illumination unit.
An object of the present invention is providing an induction coil core operable in a comparatively low frequency range (50 kMz to 1 MHz inclusive) and an illumination unit using such a core.
Another object of the present invention is providing a polycrystalline ferrite usable as the material of the above induction coil core.
The induction coil core of the present invention is a core used for an induction coil of a discharge lamp, the discharge lamp including: a bulb containing discharge gas inside; and the induction coil for generating an electromagnetic field with a frequency in a range of 50 kHz to 1 MHz inclusive in the bulb, wherein the core is made of a Mnxe2x80x94Zn polycrystalline ferrite and has a Curie temperature of 270xc2x0 C. or more.
In one embodiment, the Curie temperature is 290xc2x0 C. or more.
In another embodiment, the Mnxe2x80x94Zn polycrystalline ferrite includes Fe, Mn, and Zn where Fe element occupies 72 wt. % or more of all elements excluding oxygen.
In still another embodiment, the Mnxe2x80x94Zn polycrystalline ferrite includes Fe, Mn, and Zn and also includes Ni as an additive.
In still another embodiment, the core has a characteristic of being able to receive a magnetomotive force of 600 ampere-turn or more under the conditions of room temperature, a frequency of 100 kHz in the electromagnetic field, and a cross-sectional area of a face of the core vertical to a direction of magnetic flux of 120 mm2.
The illumination unit of the present invention includes: a bulb containing discharge gas inside; an induction coil for generating an electromagnetic field with a frequency in a range of 50 kHz to 1 MHz in the bulb; and a power supply for supplying power to the induction coil, wherein the induction coil includes a core and a winding, and the core is made of a Mnxe2x80x94Zn polycrystalline ferrite and has a Curie temperature of 270xc2x0 C. or more.
In one embodiment, the Curie temperature is 290xc2x0 C. or more.
In another embodiment, the induction coil has a characteristic of being able to receive a magnetomotive force of 600 ampere-turn or more under the conditions of room temperature, a frequency of 100 kHz in the electromagnetic field, and a cross-sectional area of a face of the coil vertical to a direction of magnetic flux of 120 mm2.
In still another embodiment, the induction coil is placed in a cavity in the center of the bulb formed by deforming a portion of an outer wall of the bulb into a concave shape, a phosphor is applied to an inner wall of the bulb, and the discharge gas includes at least rare gas.
In still another embodiment, the illumination unit is a self-ballasted fluorescent lamp.
The polycrystalline ferrite of the present invention includes Fe, Mn, and Zn where Fe element occupies 72 wt. % or more of all elements excluding oxygen, the ferrite having a Curie temperature of 270xc2x0 C. or more.
Alternatively, the polycrystalline ferrite of the present invention includes Fe, Mn, and Zn and also including Ni as an additive, the ferrite having a Curie temperature of 270xc2x0 C. or more.
In one embodiment, Fe element occupies 60 wt. % or more of all elements excluding oxygen.